Re: 2.6.35-rc1 regression with pvclock and smp guests

[Andre Przywara <andre.przywara@amd.com>]

Andre Przywara wrote:
> Avi Kivity wrote:
>>   On 07/27/2010 04:48 PM, Andre Przywara wrote:
>>>> Wierd.  Maybe the clock goes crazy.
>>>>
>>>> Let's see if it jumps forward alot:
>>>>
>>>>          } while (unlikely(last != ret));
>>>> +
>>>> +       {
>>>> +            static u64 last_report;
>>>> +            if (ret > last_report + 10000) {
>>>> +                    last_report = ret;
>>>> +                    printk("kvmclock: %llx\n", ret);
>>>> +            }
>>>> +
>>>> +       }
>>>>
>>>>          return ret;
>>>>   }
>>>>
>>>> Worth updating the 'return last' to update ret and goto the new code, 
>>>> so we don't miss that path.
>>> Did that. There is _a lot_ of output (about 350 lines per second via 
>>> the 115k serial console), both with smp=1 and smp=2.
>>> The majority is differing about 2,000,000 (ticks?), but a handful of 
>>> them are in the range of 20 million. 
>> nanoseconds.  So 2-20ms.  Consistent with 350 lines/sec.
>>
>>> No difference between smp=2 and smp=1.
>>> I also get some "BUG: recent printk recursion!" and I don't see any 
>>> kernel boot progress beyond outputting the BogoMIPS value.
>> Right, printk() wants the time too.
>>
>>> BTW: I found two message from your earlier debug statement:
>>> [    0.000000] kvm-clock: cpu 0, msr 0:1ac0401, boot clock
>>> [    0.000000] kvm-clock: cpu 0, msr 0:1e15401, primary cpu clock
>> Those are from kvmclock initialization, not from the older patch.
>>
>> I'm completely confused, everything seems to be in order.
>>
>> Let's see.  if you s/return last/return ret/ in the original, does this 
>> help things along?  this makes pvclock drop the computation and should 
>> be exactly the same as before the patch.
> Yes, this works, both smp version boot. I see a short very short break 
> after the line in question, but then it proceeds well.
> Thanks for your help, now I got a much better insight into the issue. I 
> will see if I can find something more.
Did some more investigations, some observations:
- The cmpxchg does not seem to be a problem, I didn't see the loop 
iterated more than once.
- Turning off printk-timestamps makes the bug go away. But I guess it is 
just hiding or deferring it, and it's no real workaround anyway.
- I instrumented the "if (ret < last) return last;" statement, when the 
kernel hangs I get only printks from there, although it has hit before:
----------
[    0.820000] Serial: 8250/16550 driver, 4 ports, IRQ sharing enabled
[    0.820000] returning last instead (cnt=19001)
[    0.820000] returning last instead (cnt=20001)
The last line repeats forever with the same timestamp, the counter 
(counting the number of "return last;") increments about 3500 times/second.

I will see if I find something more...

Regards,
Andre.

-- 
Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany
Tel: +49 351 448-3567-12